A haphazard approach to marketing is a waste of time and money. An occasional tweet when you have nothing better to do won’t make any difference in your sales revenue.
The first step in marketing is to develop a strategic marketing plan. And the most important components of this plan is to identify who your customers are and how you can reach them. If your book is about plumbing repairs spending time and money for advertising on website for romance readers will accomplish nothing.
I put together a spreadsheet on strategic planning. It has a series of questions, space for answers and example answers. It even has a set of anti-answers. Download a copy and start on your strategic marketing plan.